1. Field of the Invention
The present invention relates to a non-volatile semiconductor memory device for storing data using a change in resistance state of a memory element, and particularly relates to a semiconductor memory device in which a memory cell is formed by connecting the memory element and a diode in series.
2. Description of Related Art
A phase change memory device using structural change of phase change material is known as one of non-volatile semiconductor memory devices. The phase change memory device has a structure in which a resistance value of a phase change element made of the phase change material is changed by heat and thereby data can be rewritably stored. This heat is generated by flowing a large current in the phase change element. When forming a memory cell of the phase change memory device by using a MOS transistor, it is difficult to shrink the cell size in order to obtain a large gate width capable of flowing a large write current. Therefore, a phase change memory device having a diode cell structure has been proposed, in which a diode is used as a select switch for the phase change element (For example, see Patent Reference 1). For example, a phase change memory device shown in FIG. 5 of the Patent Reference 1 has a memory cell array in which a large number of memory cells having the diode cell structure are arranged in a matrix. By employing the diode cell structure, the cell size of each memory cell can be shrank, thereby achieving a high-density arrangement of the memory cells.
Patent Reference 1: Japanese Patent Application Laid-open No. 2007-134032
Generally, in a read operation of a memory cell having the diode cell structure, a predetermined voltage is applied to a selected bit line, and a current flows in a selected word line through the series connected phase change element and the diode. For example, in the phase change memory device disclosed in the Patent Reference 1, a predetermined voltage is applied to a bit line by a read circuit (corresponding to a sense amplifier 170, a bias circuit 150, a precharge circuit 160 and a clamping circuit 140 in FIG. 5 of the Patent Reference 1) driven by a boost voltage VSA of 2.5V, and in a state where a ground potential is applied to the word line, data stored in the phase change element can be determined in accordance with a change in the current flowing in the memory cell.
In order to accurately detect the change in the resistance state of the phase change element in the read operation of the phase change memory device having the diode cell structure, a forward voltage of the diode needs to be maintained constant. However, since current-voltage characteristics of the diode actually has a temperature dependence, the forward voltage of the diode fluctuates at low and high temperatures, which causes a risk that determination accuracy of the resistance state of the phase change element deteriorates. On the other hand, if a supply voltage is stepped down, it is possible to generate a voltage at which the temperature dependence of the diode is cancelled. However, in the configuration example of the Patent Reference 1, the boost voltage VSA applied to the bit line is drastically decreased when assuming the supply voltage of about 1 to 1.5V, and therefore voltage margin (sensing margin) of the read operation of the memory cell cannot be obtained. That is, if the bit line voltage is decreased in a state where the word line is maintained at the ground potential, the voltage applied to the phase change element becomes small, and it becomes difficult to configure the read circuit for detecting the current change. In this manner, in the conventional phase change memory device having the diode cell structure, there is a problem that the temperature fluctuation cannot be suppressed by cancelling the temperature dependence of the diode while obtaining the margin of the read operation.